The present invention relates to acrylic fiber which is used for production of preoxidized fiber or carbon fiber (including graphite fiber). Hereunder such acrylic fiber is referred to as a acrylic fiber precursor.
In order to produce high-strength carbon (including graphite) fibers from an acrylic fiber precursor, it be generally required that the precursor be preoxidized in an oxidizing atmosphere at 200.degree.-300.degree. C., followed by the carbonization (or graphitization) of the prexidized precursor in an inert gas atomsphere such as nitrogen gas atmosphere, at a temperature of 500.degree. C. or higher (these methods are disclosed, for example in U.S. Pat. Nos. 4,069,297; 4,543,241 and 4,536,448). In this instance, it is important that the filaments of the fiber strand being preoxidized at 200.degree.-300.degree. C. are prevented from coalescing to each other. In order to meet this requirement it has been proposed that a variety of silicone-based oiling agents are applied to the filaments during the process of production of an acrylic fiber precursor. Generally, filaments are produced by extruding a soulution of an acrylic polymer in an organic or inorganic solvent into a coagulating bath. It has been proposed to apply a silicone based oiling agent to fiber after washing and stretching the spun filaments or after drying the fiber to increase the density of the filaments. It has also been known to apply an aminopolysiloxane-based oiling agent (Japanese Patent Publication Nos. 24136/77 and 10175/78) or a polyoxyalkylenepolysiloxane-based oiling agent to acrylic fiber (Japanese Patent Application (OPI) No. 148227/77) ("OPI" as used herein means a "published unexamined Japanese patent Application).
However, the use of an aminopolysiloxane-based oiling agent is not completely effective in preventing coalescence of the filaments during the preoxidation step. Furthermore, the agents tend to promote, rather than suppress, the breakage of filaments in the step of production of the precursor. A polyoxyalkylenepolysiloxane-based oiling agent tends to penetrate into the filaments upon preoxidation, whereby the formation of voids or other defects on the surface layer or the interior of the filament during the subsequent carbonization increases. These defects decrease, rather than increase, the strength of the preoxidized fiber or carbon fibers. In order to avoid these problems, a method of using a polyoxyalkylene-polysiloxane-based oiling agent in combination with an aminopolysiloxane-based oiling agent has been proposed, but even by this method the defects inherent in the individual oiling agents can not be completely eliminated, and therefore fully satisfactory high-strength carbon fibers have not yet been obtained.